Section 9. Pitot Static System. 1 of 6

Transcription

1 1 of 6

2 ITOT STATIC SYSTEM General The pitot static systems operate the Air Speed Indicator, Vertical Speed Indicator and the Altitude Indicator for each pilot. In addition, fitted into the Co-pilot's system, is the Air Data Module for the Autopilot and a 60 Knot pressure switch to warn the crew if the landing gear is still retracted and illuminate the LG warning light and AVAD warning. Two pitot tubes, inclined at +10 are fitted on the front of the aircraft. The static vents are situated on both sides of the intermediate structure aft of the rear cabin windows. Drain valves are incorporated in each system and these can be operated from under the aircraft below the cockpit. Standby static vents are provided inside the cockpit and may be selected by use of the selector switches on each side of the centre console beneath the instrument panel. There are two switches one for each static system. These switches are normally snap wire-locked and, if used, the appropriate corrections must be made to Airspeed & Altitude. The Co-pilot's system has two additional switches (Air Data Isolating Valves) located behind his left hand on the lower front face of the electrical cupboard. These are also snap wire-locked in their normal position and may be used to isolate the Air Data Unit should a fault occur in his itot or supply lines. ITOTS Central Warning System STD BY STATIC RESSURE Right Hand Emergency Selector Valve Altimeter Air Speed Indicator itot Heat Control ATI-ICIG IT LH IT RH O O Vertical Speed Indicator OFF Figure 1 Cockpit Locations of 6

3 itot Head Total ressur Water Bleed Valve ASI ALT Emergency Cabin Air Vents VSI itot Head Total ressur Water Bleed Valve ASI ALT Emergency Cabin Air VSI Total ressure ressure Airspeed Signal for Undercarriage ot Extended Warning Air Data Module Autopilot Sensors Vents Figure 3 of 6

4 orts Bulkhead behind Copilots seat S/BY Drain Valves for Total ressure and ressure Lines AIR DATA SYSTEM STATIC RESSURE STATIC RESSURE Left Hand Emergency Selector Valve TOTAL RESSURE OFF AC Heated OFF Air Data Module Isolation Valves DC Heated itot Head Figure 3 itot Component Locations 4 of 6

5 itot Head Heating To prevent icing of the pitots each one is fitted with a 8V DC heating resistor wound around the protruding tubes. A 115V AC heating strip is also fitted in each supporting mast alongside the pitot tubing. The power supplies to these heating elements are controlled by O/OFF switches on the 34 alpha panel with a warning system to indicate failure of circuits. To prevent overheating of the pitots when the aircraft is on the ground the power supply to the heating resistors is halved. The heating electrical circuit is shown in Figure 4 along with the controls and indications. 5 of 6

6 Right-Hand itot Head X C ITOT 3 5 Resistor Heating Strip 1 6 ITOT a b + 6 Ground/Flight Logics 4 Ground/Flight Relay 1 X C 5 ITOT ITOT itot Switch (34 Alpha anel) ITOTS + Left Hand itot Heat Warning Light (3 Alpha anel) Figure 4 itot Heating Electrics (Shown on the ground and selected OFF ) Operation Switching on the battery supplies a positive to the warning light on the 34 panel, which finds it s earth through the de-energised contact of the Current Monitoring Relay (3) and illuminates the light. When the switch (1) on the 34 panel is made, the contact (a) makes the supply to the control relay () and energises it. This allows the 115V AC to flow to the heating strip and the 8V DC to flow, through the monitoring relay (3), to the heating resistor in the pitot head. The Monitoring relay is energised by the current flow and the earth contact for the warning light is broken. The light goes out. Should a break occur in the circuit to the pitot head heating resistor the monitoring relay (3) will be deenergised and the earth contact made. This will now illuminate the ITOT light on the CW (through contact (b) of the 34 switch) as well as the switch warning light. The ITOT light will extinguish when the switch is made OFF but the amber warning light remains O on the 34 panel. ote o indication is given of failure of the AC heating circuit. When on the ground the GROUD/FLIGHT logic (6) is not made and the logic relay (4) is de-energised so that, If the pitot heat is switched O, the current has to flow through resistor (5) thus reducing the heat of the pitot head. 6 of 6